In many RF applications, there is a requirement for RF circuitry to operate in more than one frequency band. For example in the field of mobile telecommunications, mobile, or cellular, telecommunications networks may be operable in more than one frequency band and/or the frequency band(s) used by mobile telecommunications networks may vary from country to country. Accordingly, mobile, or cellular, telephones are commonly arranged for operation in more than one frequency band. In this connection, four frequency bands in current use in the mobile telecommunications market are GSM (Global System for Mobile telecommunications—approx. 890 to 960 MHz), AGSM (American GSM—approx. 824 to 849 GHz), DCS (Digital Communication System—approx. 1.71 to 1.88 GHz) and PCS (approx. 1850 to 1910 MHz).
RF signals received by, or produced by, RF circuitry normally require filtering to remove, or attenuate, any signals or signal components (typically harmonic or sub-harmonic components) in unwanted frequency bands.
Conventionally, where RF circuitry is capable of operating in more than one frequency band, a respective filter network, or filter, is employed to perform the required filtering in respective frequency bands. For example, a dual band transmitter commonly comprises two separate signal paths, each path comprising a respective power amplifier for amplifying signals in a respective frequency band. Each path further comprises a respective filter for attenuating unwanted harmonic signals around the respective frequency bands. Hence, two separate filters are required even though normally only one is in use at any given time.
For many applications, for example mobile or cellular telephones, size is an important consideration and it would be desirable therefore to reduce the amount of filtering circuitry required, preferably by eliminating the need to use more than one filter. Moreover, the arrangement described above does not lend itself for use with dual, or multi, band apparatus where a single-input, single-output amplifier (sometimes referred to as a broadband or wideband amplifier) is used to amplify signals in all operating frequency bands.
United States Patent U.S. Pat. No. 5,065,120 (Munn) discloses a bandpass filter in which a capacitance is selectively switched to ground in order to affect a change in the centre frequency of the passband response of the filter. Hence, depending on the switch setting, the bandpass response of the Munn filter can be tuned to pass signals in one or other of two selectable passbands. One limitation of the Munn filter is that it operates as a band pass filter in each mode of operation. A band pass filter response is not suitable in applications where, for example, it is desired to pass all signals up to a given frequency. Moreover, it is considered that the extent to which the centre frequency of a Munn filter could be moved between modes is relatively limited and this in turn limits the application of the Munn filter.
United States Patent U.S. Pat. No. 6,195,536 (Peckham) discloses a low pass filter in which the low pass bandwidth can be adjusted by selectably biasing a plurality of diodes. One limitation of the Peckham filter is that it always acts as a low pass filter and this is not suitable for applications where, for example, it is desired to suppress signals, such as sub-harmonics, which occur at a frequency lower than the normal operating frequency band. Moreover, the Peckham filter is only suitable for use in applications where specific and non-commonplace impedance matching conditions are met.
It would be desirable therefore to provide a filter that is capable of operating in two or more frequency bands selectively and which is more versatile than the filters described above. It would also be desirable for the filter to be suitable for use with a single-input, single output multi-band amplifier.